A physical quantity sensor detects physical quantity such as acceleration and angular speed. The sensor includes a fixed electrode and a movable portion. The fixed electrode is formed on a silicon substrate. The movable portion is displaceable in accordance with the physical quantity applied to the sensor. A movable electrode is formed in the movable portion, and the movable electrode faces the fixed electrode. The sensor detects the physical quantity on the basis of a capacitance change between the fixed electrode and the movable electrode. The sensor is disclosed in, for example, JP-A-2001-330623, which corresponds to U.S. Pat. No. 6,494,096.
However, a static electrical charge generated in the movable portion or the like provides electric noise, so that the noise transmits a detection side through the substrate. Thus, detection accuracy of the sensor is reduced. In view of this difficulty, it is considered that the sensor detects the physical quantity on the basis of intensity change of a reflected light, which is reflected on the movable portion. In this case, the sensor can detect the physical quantity with high detection accuracy. Specifically, the light is emitted toward the movable portion so that the light is reflected by the movable portion. Thus, the sensor can detect the physical quantity accurately even when the electric noise is generated in the sensor.
The above optical type physical quantity sensor is integrally formed on the silicon substrate by using a micro electrical mechanical system method (i.e., MEMS method). Specifically, the movable portion and optical parts such as a lens, a prism and an optical waveguide are integrated on the substrate. Accordingly, the sensor has small dimensions and a low manufacturing cost.
However, it is necessary to form the movable portion and the optical parts independently. Thus, a manufacturing efficiency of the sensor is low.